1. Field of the Invention
The present invention relates in general to a method of to aligning a wafer and masks. In particular, the present invention relates to a method of aligning a wafer mask wherein alignment marks are deposited in each die area.
2. Description of the Related Art
Forming an integrated circuit on a substrate (wafer) requires a series of process steps. These process steps include the deposition and patterning of material layers such as insulating layers, polysilicon layers and metal layers. The layers are typically patterned using a photoresist layer that is patterned over the material layer by exposing the photoresist through a photomask or a reticle. The photoresist is then developed to provide the pattern. Typically, the photomask or the reticle has alignment marks that are aligned to alignment marks formed on the substrate in a previous process step. However, as the integrated circuit feature size continues to decrease to obtain increased circuit density, it has become difficult to register or align one masking level to the previous level.
As shown in FIG. 1, generally, two global alignment marks 142 and 144 are deposited on the mask 140 in the conventional alignment method, and then the alignment marks 102 and 104 are aligned with the two global alignment marks 142 and 144 by a Hexe2x80x94Ne Laser of an exposure system 160. Next, all the fields, for example chip areas or die areas, on the wafer 100 are exposed in a step-and-repeat mariner using an optical stepper. Consequently, the wafer 100 usually has a plurality of fields (dies) divided by the scribe lines. A desired pattern 146 in the mask 140 is transferred to each corresponding field in the wafer by exposing through the lens 120, step-and-repeat.
In the above method, the global alignment marks 142 and 144 are formed on the mask 140 for aligning with the marks 102 and 104. Further, a plurality of alignment marks 148 are deposited on the intersection of scribe lines for intra-field alignment. Therefore, different patterns on each mask can be transferred to the fields in the wafer by global alignment and the intra-field alignment.
As shown in FIG. 2, the alignment marks 1031 are complete when the exposure field 103, for example a shot, is complete. Therefore, the field on the wafer can align with the masks accurately by the exposure system and then the optical stepper can transfer the pattern from the mask to each field 105 accurately. However, the alignment marks 148 on the scribe lines may be unclear or disappear entirely incomplete exposure fields at the wafer""s edge regions, even if the fields form complete squares. Therefore, the fields at the wafer edge regions cannot align with the masks accurately. In this case, the optical stepper cannot transfer the pattern from the mask to each field 105 accurately. This situation is illation in FIG. 2, where all the 106A, 106B, 106Cxcx9c106G at the wafer edge region may not align with the masks accurately because the alignment mark is incomplete or missing.
It is an object of the present invention to provide a method of aligning a wafer and masks. In the present invention, alignment marks are deposited at the corner of each field. Consequently, the exposure system can align the mask with each field accurately for exposure process. Furthermore, after the fields are detached by a wafer saw, the alignment marks at the corner of each field also can be used for alignment in subsequent processes, for example, bonding wire in the packing process.
In the present invention, a wafer having a surface with a plurality of fields and scribe lines is provided. An initial mask and a subsequent mask having a first pattern and a second pattern respectively corresponding to the fields are provided. The initial mask and the subsequent mask further have a plurality of original alignment marks respectively at the corners thereof corresponding to the fields. Next, the first pattern is transferred to each field and a plurality of secondary alignment marks corresponding to the original alignment marks are formed by the initial mask. Then intra-field alignment is performed by aligning the original alignment masks of the subsequent mask with the secondary alignment marks at the corner of the field to form the second pattern on each field.